Patients undergoing allogeneic stem cell transplantation for chronic myelogenous leukemia (CML) often enter complete hematologic and cytogenetic remission. Nonetheless, using sensitive techniques, even patients in stable clinical remission often have residual BCR-ABL tumor cells circulating in their blood for many years. Quantitative RT/PCR for BCR-ABL is the most sensiive technique currently available for monitoring tumor load and recognizing the early phases of tumor progression before clinical signs are apparent. In collaboration with Dr Barrett of NHLBI and his associates, we routinely use BCR-ABL RT/PCR to monitor CML disease status after transplant. A recent retrospective review of clinical results in Dr. Barrett's patients who developed molecular or clinical relapse after allotranplantation provides important new information about the relative efficacy of donor lymphocyte infusion (DLI) and the kinase inhibitor imatinib, the two most effective therapeutic modalities available. In retrospective review, the group could identify 37 relapsed patients who were treated with DLI and/or imatinib. Thirty patients in this cohort responded to therapy. Clinically, most patients receiving a combination of imatinib and DLI responded quickly (10 of 11 responded within 3 months) and all ultimately achieved a high grade remission. By comparison, 2 of 22 patients treated with single modality achieved remission within 3 months and only 11 of 22 ultimately achieved high grade remission. These results have important implications. Imatinib is very attractive as a single agent for treating CML in relapse since it has fewer potential side effects than DLI, which can sometimes induce GVHD. These studies however, clearly indicate, despite its risks, DLI still has an important role in maximizing clinical response in transplanted CML patients who experience a relapse.